1. Field of the Invention
This invention relates to a focus detecting device for a camera or the like for changing over and effecting (1) the active type focus detection using a light projection device for focus detection (including a conventional auxiliary light device) which projects a light onto an object which is the object of the focus detection for an objective lens, and (2) the passive type focus detection which does not use the light projection device for focus detection.
2. Related Background Art
As one type of focus detecting device of a camera, there is known a device in which the exit pupil of a photo-taking lens is divided into two by an optical system for focus detection and two object images formed by light beams passed through the respective pupil areas are received by a photoelectric converting element array (for example, a CCD sensor array) and the focus state of the photo-taking lens is detected from the output thereof.
Also, a device which has a plurality of focus detecting optical systems as described above and is contrived so as to extract the two-dimensional pattern of an object and which avoids the disadvantage peculiar to the aforedescribed system wherein focus detection disability is experienced when the object pattern exists only in a direction orthogonal to the photoelectric converting element array is disclosed in Japanese Laid-Open Patent Application No. 62-95511.
Further, an active type focus detecting device used in combination with light projection means for focus detection and designed so as to be capable of accomplishing focus detection even for an object under low illumination (which is a disadvantage peculiar to the passive type in which focus detection is effected passively by only the light from an object) and to project a predetermined pattern onto the object during low illumination and detect the reflected pattern image from the object to thereby accomplish focus detection is disclosed, for example, in Japanese Laid-Open Patent Application No. 62-324091.
To improve the focus detecting ability of the active type device, the assignee of the subject application has previously proposed a focus detecting optical system as shown in FIG. 9 of the accompanying drawings. Referring to FIG. 9, a field lens 2 is disposed with the same axis as a photo-taking lens 1. Field masks 3a and 3b for limiting field of view are disposed near the field lens 2. Light beams passed through the field masks 3a and 3b are wavelength-limited by optical filters 4 and 5, respectively. The optical filter 4 is an infrared cut filter for cutting the infrared light, and the spectral characteristic thereof is as shown in FIG. 11 of the accompanying drawings. The purpose of inserting this infrared cut filter is to prevent the infrared aberration of the photo-taking lens 1 from adversely affecting the focus detecting system. Also, the optical filter 5 forms a band-pass filter for transmitting therethrough only the wavelength of the emitted light of the light emission source of a light projection system which will be described later.
Rearwardly of the field lens 2, two secondary imaging lenses 6a and 6b are disposed at positions symmetrical with respect to the optic axis. Rearwardly of the secondary imaging lenses, there are disposed photoelectric converting element arrays 7a and 7b on which the image of the field mask 3a is formed and photoelectric converting element arrays 8a and 8b on which the image of the field mask 3b is formed.
This example is used in combination with a light projection system for focus detection as shown in FIG. 10 of the accompanying drawings, and the light projection system for focus detection projects a pattern light onto an object. In FIG. 10, the reference numeral 10 designates a projection light source such as a light emitting diode, and the light beam projected therefrom illuminates a pattern member 12 having an irregular chart shape through a relay lens 11 for illumination. A focus detecting device of the pupil-division type is liable to malfunction for a synchronous pattern and therefore, a random pattern is preferable. The random chart of the illuminated pattern member 12 is projected onto the surface of an object by a light projection lens 12. The imaging relationship between the pattern member 12 and the surface of the object need not be very strict. The projected pattern need not always be represented by a chart shape, but, for example, the pattern may be prescribed by the shape of the light emitting portion of the projection light source 10.
Now, it is desirable that as the projection light source 10, use be made of a light emission source of reduced visibility so as to avoid the infrared aberration of the photo-taking lens 1 and not to dazzle the eyes. Also, it is popular from a practical viewpoint such as size to use a light emitting diode having such a spectral strength distribution as shown in FIG. 12 of the accompanying drawings.
FIG. 13 of the accompanying drawings shows the photoelectric conversion signal outputs of the photoelectric converting element arrays 7a, 7b and the photoelectric converting element arrays 8a, 8b when the object has no contrast and a pattern light is projected by the light projection system for focus detection shown in FIG. 10. FIG. 13A shows the signal outputs of the photoelectric converting element arrays 7a, 7b, and FIG. 13B shows the signal outputs of the photoelectric converting element arrays 8a, 8b. A signal processing method for detecting the amount of image deviation PR from the signal outputs of the photoelectric converting element arrays 7a, 7b and the photoelectric converting element arrays 8a, 8b is disclosed in Japanese Laid-Open Patent Application No. 58-142306, Japanese Laid-Open Patent Application No. 59-107313, Japanese Laid-Open Patent Application No. 60-101513 or Japanese Patent Application No. 61-16082. The wavelengths of all visible light enter the photoelectric converting element arrays 7a and 7b and therefore, the energy of outside light and the reflected energy of the projected light beam are mixed together and the light projection pattern is compressed as shown in FIG. 13A. The degree of compression is governed by the intensity of the projected light energy, the object distance and the intensity of the outside light.
On the other hand, in the photoelectric converting element arrays 8a and 8b wavelength-selected by the projection light source 10, a projected light pattern is extracted as shown in FIG. 13B. This is because the rate of the outside light is decreased relative to the reflected light of the projected light pattern and therefore the S/N ratio of the signal is improved. If the S/N ratio of the signal to the outside light is improved, focus detection will become possible up to a greater distance even when the outside light is bright.
Further, the assignee of the subject application has also previously proposed another method of discriminating between a projected light beam and outside light. That is, this previously proposed device has electrical means for modulating a projection light source in terms of time and selectively discriminating the electrical output portion of a photoelectric converting element array produced by the contribution of the projection light source from the portion by outside environmental light, and the defocus amount of a photo-taking lens is detected by the use of the photoelectric conversion output of the optical image by the discriminated projection light source.
The electrical construction of the above-described previously proposed device is diagrammatically shown in FIG. 14 of the accompanying drawings. The oscillation circuit 15 of a light projection device 14 provides its output to a projection light source 17 through a resistor 16. The oscillation frequency of the oscillation circuit 15, i.e., the modulation frequency of the projected light beam of the projection light source 17, is not specially limited, but if the frequency is too low, the responsiveness of the focus detecting device will be reduced and further, the capacity used for an AC amplifier which will be described later will become great, and this may cause a difficulty in making an integrated circuit. If the frequency is too high, it will become impossible to construct a linear amplifier.
In FIG. 14, the portion other than the light projection device 14 shows a focus detecting device 18. Photoelectric converting element arrays 19a and 19b comprise a plurality of photoelectric converting elements 19a.sub.1 -19a.sub.n and 19b.sub.1 -19b.sub.n, respectively, and the photoelectric conversion outputs of the individual photoelectric converting elements are discriminated from the outside environmental light and made into direct currents by discrimination DC circuits 20a.sub.1 -20a.sub.n and 20b.sub.1 -20b.sub.n, whereafter they are output to a common output bus 23 through switches 22a.sub.1 -22a.sub.n and 22b.sub.1 -22b.sub.n which are clocked by a shift register 21. The photoelectric conversion outputs are time-serially sampled by an analog digital converter AD in a microcomputer 24 and are successively stored in a memory RAM for data. CPU designates a calculating unit, ROM denotes a memory for the operating program, and Vref designates a constant voltage.
The discrimination and DC-conversion of each photoelectric converting element will hereinafter be described in greater detail with reference to FIG. 15 of the accompanying drawings. In FIG. 15, only the photoelectric converting element 19a.sub.1 is shown, but the other photoelectric converting elements are similar thereto. The anode side of the photoelectric converting element 19a.sub.1 is common-wired to a constant voltage source which outputs the constant voltage Vref, and the cathode side thereof is connected to the inverting input terminal of a current-voltage converting amplifier 25. The voltage output of the current-voltage conveting amplifier 25 is input to and amplified by a non-inverting amplifier 27 through a coupling capacitor 26 for removing a DC component. Generally, the optical image of a daytime outdoor object irradiated with the sun has no variation with time and is a DC component and therefore is removed by the coupling capacitor 26. Also, an object under artificial illumination is subjected to modulation by the frequency of the commercially available power source, but is likewise removed because it is a very low frequency. That is, in the present device, it is necessary that the time constant determined by the input impedances of the coupling capacitor 26 and the non-inverting amplifier 27 be the time constant of such a high-pass filter that there is a sufficient gain at the modulated frequency of a projected light beam but the gain is sufficiently low at the frequency of the commercially available power source. The reference numeral 28 designates a wide-band half wave rectifier which makes the photoelectric conversion output of the optical image of an amplified projected light beam component into a direct current. The reference numeral 29 denotes a mirror integration circuit having a time constant, which smoothes the half wave rectification output. The time constant of a time constant circuit 30 must be set to a time sufficiently longer than the inverse number of the modulated frequency of the projected light beam and short enough so that no inconvenience is felt in operating the camera. The photoelectric conversion output of the object image formed by the projected light beam which has been made into a direct current is output to the common bus 23 through a switch 22a, controlled by the shift register 21.
The optical image photoelectric conversion output having the portion contributed to the outside environmental light cut by the discrimination DC-making circuit of the construction as described above is successively written into the memory RAM by the calculating unit CPU. After a sequence of data sampling, the amount of relative displacement (the so-called amount of image deviation) of two images formed by the imaging light beams passed through different pupil areas of the photo-taking lens is calculated by a conventional calculating method, and is converted into a defocus amount. If the converted value is within the in-focus judgment standard, in-focus is judged, and if the converted value is outside said standard, the photo-taking lens is driven on the basis of the calculated defocus information.
Where there are focus detection of the passive type and focus detection of the active type, focus detection of which type would be effected will hereinafter be described with respect to a focus detecting device having, for example, in addition to focus detection of the passive type, focus detection of the active type which does not have means for discriminating between the outside light and the projected light pattern. Heretofore, in such a case, the active type had only an auxiliary role for the passive type. The manner in which the two types are used will now be described with reference to the flow chart of FIG. 16 of the accompanying drawings.
First, at #1, photoelectric charge accumulation by the passive type is effected without the use of the light projection device. This is because it is better to effect focus detection by a system advantageous as much as possible in terms of energy conservation and focus detection of the active type is used auxiliarily. When the photoelectric charge accumulation of the passive type is terminated, at #2, the output of a photoelectric converting element array is introduced into a memory for data in a microcomputer, and the amount of image deviation on the photoelectric converting element array is calculated to thereby find the amount of driving of the photo-taking lens. At this time, the degree of coincidence between the two images, the contrast and the luminance of the images are calculated at a time. Subsequently, at #3, whether this focus detection result is effective is judged from the degree of coincidence between the images, the contrast and the luminance of the images calculated at #2. When it is judged that the focus detection result is good, at #4, in-focus is judged, and depending on the result thereof, lens driving (#5) or in-focus display and release operation, not shown, are effected. When at #3, the focus detection result is not effective, at #6, the light projection device for focus detection is operated to effect the photoelectric charge accumulation of the active type. When the accumulation in the photoelectric converting element array is terminated by the reflection of the projected light, at #7, focus detection calculation is effected, and at #8, the effectiveness is judged. If the focus detection result is not effective, focus detection is impossible and therefore the display of it is effected or search is effected. If the focus detection result is effective, at #4, in-focus is judged and similar control to that during the focus detection of the passive type is effected.
In the aforedescribed previously proposed example, no consideration has been given to the passive type and the active type in spite of there being objects easy to be focus-detected by these two types and objects difficult to be focus-detected by these two types, and in spite of the fact that selection of the two types is an important matter, and the passive type has been chiefly used and the active type has merely played an auxiliary role therefor. Particularly, in the active type wherein the means for discriminating between the projected light beam and the outside light is absent, even in an object situation in which the active type is clearly advantageous, focus detection of the active type is not effected until it is judged that the result by the photoelectric conversion output of the passive type indicates the impossibility of focus detection, and this has led to the disadvantage that the responsiveness of the system is bad when the active type is selected. Further, in the case of the active type which has the means for discriminating between the projected light beam and the outside light, it is often the case with such auxiliary use of the active type that even for an object for which the active type is advantageous, focus detection is effected in the passive type, and this becomes disadvantageous in terms of accuracy and therefore, it has been necessary to select the active type and the passive type by some method or other.
For example, assuming that a dark object for which the passive type is difficult has been focus-detected, the accumulation time at #1 in FIG. 16 is set long. Therefore, at #3, it is judged that the focus detection result is bad, and much time is taken until the accumulation of the active type of #6 is reached, and responsiveness becomes bad.
Also, if the contrast of an object itself is very low in spite of there being a quantity of light to some degree, there may be not only a case where the photoelectric conversion output obtained from the photoelectric conversion element array is such as shown in FIG. 17A of the accompanying drawings, but also a case where said photoelectric conversion output is such as shown in FIG. 17B of the accompanying drawings Such an object is awkward to the focus detection of the passive type, and there is the disadvantage that if as a result of the focus detection of the passive type, the active type is selected after focus detection has become impossible, the responsiveness of the entire focus detecting operation will become bad.
Also, there is a case where even for an object unsuitable for the passive type, as compared with the active type, the focus detection result is judged as being effective at #3 and focus detection worse in accuracy than when focus detection is effected in the active type is effected.